The present invention relates to an alternating voltage amplitude detector, more particularly to a low power signal detector comprising metal-oxide-semiconductor field-effect transistors.
An access control system as shown and described in U.S. Pat. No. 4,353,064, entitled "Battery Operated Access Control Card," assigned to the assignee of the present invention, utilizes an access control card including a receiving circuit coupled to an antenna for receiving radio frequency signals through space. The aforementioned patent utilizes an arithmetic shift key (ASK) approach as a means to transfer coded information from the transmitter to the access card. Such a card contains a low power battery and therefore necessitates a low power signal detector to keep power consumption at a minimum particularly when not in use. Further, such a signal detector should have a threshold minimum below which signal is not detected, and an access delay time for responding to an RF signal. A threshold detector coupled to the output of the signal detector can then provide pulses of information to subsequent signal processing circuits for code checking and the like in an access control system.
The receiver portion of the circuit needs to be powered up continuously or by continuous sampling so as to be responsive when the card is in actual use for obtaining access to a controlled security area. Therefore, it is imperative that such a receiver be constructed of a design which is of very low power consumption. Metal-oxide-semiconductor field-effect transistors are well known low power devices. They are particularly of low power when the MOS transistor is reduced below the threshold voltage defined by the usual strong inversion characteristic. In this mode of operation, the channel current is an exponential function of the gate-source voltage--i.e. below the saturated channel current level. An analysis of the weak inversion mode operation of MOS transistors is taught in a paper entitled, "CMOS Analog Integrated Circuits Based On Weak-Inversion Operation," by Vittoz et al., IEEE Journal of Solid State Circuits, Volume SE-12 No. 3, June 1977.